Concave-Up Pelecypod (Psephidia) Shells in Shallow Marine Sand, Elk River Beds, Southwestern Oregon

Abstract

ABSTRACT The Elk River Beds of Pliocene or Pleistocene age in southwestern Oregon contain numerous beds in which disarticulated pelecypod valves, particularly of the small pelecypod Psephidia, lie predominantly concave-up. Inasmuch as the trait seems to result from deposition in a wave-swept shallow marine environment, this shell configuration cannot here be attributed to mechanisms such as turbidity currents, sediment traps, uppermost beach swash, or deposition in quiet water that have been previously cited to account for concave-up shells at other places. Experiments with Psephidia shells in the surf zone indicate that layers of concave-up shells are formed by migrating small sand ripples on the sea floor. Such ripples may produce much of the lamination found in the Elk River B ds and in other shallow marine deposits.